Light-modulating medium for image projection apparatus

ABSTRACT

Addition of small amounts of substituted anthraquinones to a light-modulating fluid substantially reduces the rate of substrate formation in the fluid thereby substantially increasing its useful lifetime and consequently the useful lifetime of the light valve of the projection apparatus in which the modified fluid is used as the light-modulating medium.

States Patent [1 1 [111 3,764,59 Orser Oct. 9, 1973 LIGHT-MODULATING MEDIUM FOR 3,288,927 11/1966 Plump 178/7.5 D

IMAGE PROJECTION APPARATUS 3,274,565 9/1966 Wright 178/7.5 D

Inventor: David A. Orser, Liverpool, N.Y.

General Electric Company, Schenectady, N.Y.

Filed: Jan. 11, 1973 Appl. No.: 322,639

Assignee:

U S. Cl 252/300, 178/7 5 D, 313/91 Int. Cl. G01b 5/24 Field of Search 252/300; 313/91;

References Cited UNITED STATES PATENTS 1/1973 Perlowski, Jr. 178/75 D Primary Examiner-Norman G. Torchin Assistant Examiner-J. P. Brammer Attorney-James W. Underwood et al.

[57] ABSTRACT 7 Claims, No Drawings LIGHT-MODULATING MEDIUM FOR IMAGE PROJECTION APPARATUS This invention relates to an improved fluid, lightmodulating medium for the projection of self-erasing, rapid decay images in apparatus of the kind wherein a a fluid, light-modulating medium is deformed into diffraction and/or refraction gratings by the impression of electron charges thereon as a function of electrical signals corresponding to the images. By the use of these improved fluids, improved light valves for electronic projection systems are provided which have an increased lifetime during which they are capable of projecting an acceptable image.

Apparatus employing such a fluid, light-modulating medium is described, for example, in U.S. Pat. No. 2,943,147 Glenn, .lr., wherein is disclosed a projection system having a light valve in the form of an evacuated glass envelope containing an electron gun for producing an electron beam, which is deflected in a reactangular raster over the surface of a light-transmitting, electron-deformable, light-modulating medium contained within a portion of the transparent glass envelope. The electron beam is modulated, as by a television signal applied to the deflection means. The deflected electrons strike a portion of the lightmodulating medium in the raster area traversed by the electron beam, and these electrons are electrically attracted to a conductive coating over which the lightmodulating medium is supported. As these electrons strike the surface of the medium, they produce deformations in this surface with the amplitude of the deformations being a function, along with other parameters, of the number of electrons deposited by the electron beam at various points over the surface of the raster area. As a result, the amplitudes of these deformations are a function of the electron beam modulation. Repetitive rates of more than one image per second are employed and this is possible because of the rapid decay of each image such that the same area of fluid is employed for the formation of a quick succession of images.

Conventional deformable light-modulating mediums are described in the aforesaid Glenn patent and improved mediums are described in U.S. Pat. Nos. 3,288,927 Plump, 3,317,664 and 3,317,665 Per lowski, Jr., and 3,541,992 Herrick et al. All of these patents are hereby incorporated by reference for a discussion of the properties required of the lightmodulating fluid and for a showing of the type of apparatus and a description as to how it operates in projecting an image utilizing either the prior art, fluid, lightmodulating medium or the fluid, light-modulating medium of the instant invention. The actual operation of the apparatus and the way in which these fluids are employed therein does not form a part of the instant invention, except to the extent that the improved properties of my fluids result in increasing the useful lifetime of the light valves and thereby provide an improved projection apparatus in which they are used, which is more fully disclosed and discussed hereinafter.

The fluid, light-modulating medium is a thin layer of light-transmittable fluid in which the electron beam forms phase diffraction gratings and/or refraction gratings in the form of alternate hills and valleys caused by the deforming effect of the electron beam. The adjacent valleys are spaced apart by a predetermined distance such that each portion of light incident on a respective small area or point of the medium is deviated in a direction orthogonal to the direction of the valleys. The intensity of the deviated light is a function of the depth of the valley and diminishes as auto-erasure of the deformation occurs wherein the hills and valleys just diminish and the fluid is ready for the writing of a new image.

Under continued use, the fluid suffers some radiation damage due to the continued exposure to the electron beam. One of the effects of this damage is that a substrate is deposited on the transparent solid surface through which the light beam passes. This substrate layer is darker colored, has a different refractive index than the solid surface'on which it is deposited and is not deposited uniformly over the entire raster area traversed by the electron beam. The light beams which are modulated by the fluid are affected to different degrees due to these characteristics of the substrate layer. The net result is that the acuity of the projected image is adversely affected. This situation grows progressively worse as the amount of substrate increases until finally the light valve must be replaced in order to provide an acceptable projected image.

Although improvements have been made in providing improved radiation-resistant fluids and in improving their lifetime and writing characteristics, still further improvements are desirable to extend the lifetime of these fluids and, therefore, the lifetime of the light valves containing these fluids still further.

It is therefore a prime object of this invention to provide light-modulating fluids having considerably improved radiation resistance and which are resistant to the formation therein of substrates or products which detract from their usefulness in producing images.

It is also an object of this invention to obtain the additional advantages to be gained from increased useful lifetime of the light-modulating fluids whereby the useful lifetime of the light valve in the projection system will be increased.

It is another object of this invention to provide an improved light-modulating fluid yielding greater acuity in the projection system.

These objects and other advantages are obtained by using certain anthraquinones as stabilizers for the lightmodulating fluids. Although my invention is applicable to any of the particular light-modulating fluids described in the above Herrick U.S. Pat. No. 3,541,992, the one fluid which possesses the best combination of desirable properties for a light-modulating fluid is polybenzyltoluene. As further disclosed by Herrick, various polymers can be dissolved in these base fluids to improve their viscoelastic behavior. The preferred polymer is polystyrene. The polybenzyltoluene fluid can be the polybenzyltoluene fluid obtained by condensing benzyl chloride with toluene in the presence of a Friedel-Crafts catalyst to produce a fluid having a viscosity of at least centistokes (cs.) at 25C. which is separated from the lower boiling fractions by distillation. Such a fluid is described in the above-referenced Plump U.S. Pat. No. 3,288,927. A preferred and improved polybenzyltoluene is prepared by condensation of benzyl alcohol with toluene in the presence of an acidic catalyst as described in the copending application of Charles E. Timberlake, Ser. No. 92,178, filed Nov. 23, 1970 and assigned to the same assignee as the present invention. This material, likewise, is distilled to provide a fraction having a viscosity of at least 100 centistokes at 25C.

These fluids can be modified by dissolving an amount of polystyrene sufficient to attain the desired viscoelastic behavior in the fluid. Simultaneously, or as a separate step, tris(3-phenylpropyl)phosphate can be dissolved in the fluid to improve its critical quieting thickness (CQT) as disclosed and claimed in my copending application, Ser. No. 254,696, filed May 18, 1972 and assigned to the same assignee as the present invention. The amount of tris(3-phenylpropyl)-phosphate is determined by the properties one desires to obtain. Very small amounts in the order of 0.ll percent will have some effect on increasing the CQT of the fluid but still further gains are obtained by use of still larger amounts. However, the further gain in CQT must be measured against the fact that larger amounts of the tris-(3- phenylpropyl)phosphate decrease the viscoelastic nature of the fluid. This can be compensated to some extent by increasing the amount of polystyrene added or the molecular weight of the polystyrene used can be increased. To obtain a balance between the requisite viscoelastic behavior and the desired increase in CQT, amounts up to about 6 percent can be used but the best all-around balance of desirable properties are usually obtained at a concentration of about 4 percent by weight. However, these percentages are not fixed and vary dependent upon the viscosity of the particular polybenzyltoluene used, both the molecular weight and degree of branching of the polystyrene used and the amounts of polystyrene dissolved in the polybenzyltoluene fluid. Therefore, the amount of polystyrene dissolved in the polybenzyltoluene is best described as that amount sufficient to provide viscoelastic behavior to the fluid and the amount of tris (3- phenylpropyl)phosphate is best described as the amount sufficient to increase the critical quieting thickness of the fluid but insufficient to destroy the viscoelastic behavior of the fluid. There is no need to use larger amounts of either of these materials than is required to obtain the desired properties.

I have found that a wide variety of anthraquinones can be used as stabilizers for these fluids to decrease substrate formation. However, some have very low so]- ubility in the fluid and therefore can crystallize out as the fluid cools when the apparatus is not in use and still others have a vapor pressure sufficiently high that they distill or migrate from the relatively warm fluid to cooler parts of the light valve. Only very small amounts of the anthraquinones have to be used to provide the desired stability and substantially decrease the rate of substrate formation. Amounts as low as 0.01 percent are very effective with further improvement being obtained with increasing amounts. Generally, there is no need to use amounts greater than 0.5 to 1 percent, although larger amounts can be used up to the solubility limit of the particular anthraquinones if the increased amount does not seriously affect any of the other desirable characteristics of the fluid, e.g., the viscoelastic behavior, CQT, etc.

Basically, the only requirements of the anthraquinones are that they (1) be sufficiently soluble in the fluid to provide the desired concentration without crystallizing in use, (2) have sufficiently low vapor pressure when dissolved in the fluid that they do not migrate from the liquid under the temperature and pressure of the light valve, (3) do not have substituents that are sensitive to electron radiation, and (4) do not deleteriously affect the functional properties of the base fluid to a degree that it can not be tolerated in the fluid and still have the requisite light-modulating properties.

Anthraquinone and its lower alkyl derivatives have too high a vapor pressure and/or are not radiation resistant. However, I have found that phenyl and benzyl substituents are radiation resistant and greatly reduce the vapor pressure of anthraquinone. I have found that only one such substituent needs to be present to reduce the vapor pressure sufficiently that it can be used for my purpose. However, two such substituents are even more effective. Any further increase in the number of such substituents adversely affects the solubility in the fluid. Thest two substituents can be either directly on the ring or the second can be a substituent on the first, i.e., the anthraquinones can be phenyl-, benZyl-, diphenyl-, dibenzyl-, phenyl-benzyl-, biphenylyl, (phenyl)- benzyl-, (benzyl)phenylor (benzyl)benzylanthraquinones. Mixtures of any two or more of these can be used. These anthraquinones have the formula where R, is phenyl, C H benzyl, C H CH (pyenyl)benzyl, C H C H CH benzylphenyl, C H CH C H (benzyl)benzyl, C H CH C H CH or biphenylyl, C H C H and R is the same as R and in addition hydrogen. R and R can be on either one or both aromatic rings, but not the quinoid ring, i.e. be on positions I, 2, 3, 4, 5, 6, 7 or 8 but not 9 or 10.

Because they are the easiest to make in high yields and high purity and because of their excellent performance as stabilizers to decrease substrate formation in the light-modulating fluids, the preferred anthraquinones are the diphenylanthraquinones, which are readily available as the 1,4-; 2,3- and 1,5-isomers, the benzylanthraquinones, either the l-, or 2-isomer or mixtures thereof, with the Z-benZyI-anthraquinone being the easiest to obtain pure and the (benzyl)benzylanthraquinones with the 2-(p-benzyl)benzylanthraquinone being the easiest to prepare. The benzylanthraquinones and (benzyl)benzyl-anthraquinones are new chemical compounds and are disclosed and claimed in my copending application, Ser. No. 322,636 filed Jan. 1 1,1973 and assigned to the same assignee as the present invention.

Resistance to substrate formation is readily determined in an accelerated test by placing a portion of the fluid on a segmented disc in an evacuated chamber and bombarding with electron radiation from an 8 KV source. In order to assure comparable results, a blank is also included with the various samples, each on its own segment and the disc rotated under the electron beam whose vertical and horizontal sweep is controlled to give a 0.3 inch by 0.3 inch raster area. After irradiation for a designated time, the segments are washed free of the test fluid, the substrate, if any, observed, and the loss in transmission of the blue component of light measured photometrically. This value is expressed as percent blue transmission loss (BTL) and is a measure of the amount of the blue component of white light that is absorbed by the substrate. The ideal situation (for maximum efficiency) would be to decrease blue transmission loss to zero. The blue transmission loss is thus a measure of substrate formed in a particular fluid. The reported values are the average of the maximum values found in at least three separate traversals of the irradiated area. Comparison of the amounts of substrate formation in a light valve and in this test, as well as theoretical calculations show that the rate of substrate formation is three times as fast in this accelerated test.

In order that those skilled in the art may better understand my invention, the following examples are given by way of illustration and not by way of limitation. In the examples the polybenzyltoluene fluids are designated PBT-A and PBT-P to indicate the fluids made by AlCl and polyphosphoric acid catalysis respectively. When these fluids were modified with polystyrene, the amount of the latter was 2 percent by weight and the fluids are designated as PBT-A/PS or PBT-P/PS. The following abbreviations are also used: PPP tris(3-phenylpropyl)phosphate, DPA 1,4- diphenylanthraquinone, BA 2-benzylanthraquinone, DBA 2-(p-benzyl)-benzylanthraquinone. The reported times are in hours of exposure of the segmented disc to the electron beam. Percentages of additives are by weight.

Fluid Quinone BTL EXAMPLE 1 Time 87.4 hours PBT-P 0.55 do 0.01% DPA 0.46 do 01% DPA 0.19 PBT-P/PS 0.53 do 0.1% DPA 0.43

EXAMPLE 2 Time 100.8 hours PBT-P 1.11 PBT PIPS 1.18 do 0.01% BA 0.65 do 0.5% BA 0.28

EXAMPLE 3 Time 147 hours PBT-A/PS 4.81 do 0.5 DBA 0.19 PBT-P/PS 1.31 1% PPP do 0.2 DBA 0.85

EXAMPLE 4 Time 527 hours PBT-P/PS 1.57 do 0.05 DBA 0.89 do 0.1 DBA 0.66 do 0.2 DBA 0.46

In all of the above fluids, the overall writing characteristic of a particular fluid was not significantly changed by the inclusion of the quinone.

Two light valves, each having as its fluid, PBT-P/PS containing 0.2 percent DBA have been in continuous operation on a life test. One has accumulated 4,806 hours and the other 2,356 hours of operation with no visibly detachable substrate formation.

From the above examples, it will be readily apparent that the blue transmission loss of each of the materials of the present invention is far and away less than that of a typical prior art material. These results are indeed surprising and unexpected since H.A. Hartzfeld and RB. Regier report in A.E.C. Research and Developstokes at 25C. in which there is dissolved a stabilizing amount of an anthraquinone having the formula where R is phenyl, benzyl, (phenyl)benzyl. benzylphenyl, (benzyl)benzyl or biphenylyl, R is the same as R and in addition hydrogen and R and R can be on either one or both aromatic rings.

2. The fluid of claim 1, wherein the anthraquinone is Z-benzylanthraquinone.

3. The fluid of claim 1, wherein the anthraquinone is 2-(p-benzyl)benzylanthraquinone.

4. The fluid of claim 1, wherein the anthraquinone is l,4-diphenylanthraquinone.

5. The fluid of claim 1, in which the polybenzyltoluene is a polystyrene-modified polybenzyltoluene.

6. In a fluid, light-modulating medium for use in apparatus in which a thin layer of said fluid, lightmodulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said plane to build up charge in said fluid, light-modulating medium, which charge produces self-erasing deformation in the surface of said layer, each said deformation acting to diffract light directed at said layer from a light source in a light optical system, the diffracted light being projected by the optical system as a function of each deformation to form self-erasing, rapid, decay images, the improvement wherein said fluid is the fluid of claim 1.

7. in a fluid, polystyrene-modified polybenzyltoluene light-modulating medium for use in apparatus in which a thin layer of said fluid, light-modulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said plane to build up charge in said fluid, light-modulating medium, which charge produces self-erasing deformation in the surface of said layer, each such deformation acting to diffract light directed at said layer from a light source in a light optical system, the diffracted light being projected by the optical system as a function of each deformation to form selferasing, rapid, decay images, the improvement wherein the fluid, polystyrene-modified polybenzyltoluene light-modulating medium is the fluid of claim 1.

Patent No. 3,7 l-a5 -l- Dated October 1 1973 Inventor(s) David Av P G-I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column L line 30, "(pye-" should read (phe- Signed and sealed this lL th day of May 197L (SEAL) Attest:

EDWARD l immmmfi, JR. i 0. MARSHALL DAN N Attesting Officers Commissioner of Patents FORM PC4050 uscoMM-oc wan-Pu Q ".5. GQVERNMENT PRINTING oI'l'lCE 2 I. v-'SI.-$I"

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,7 b5 l-9 Dated October 9, 1973 Inventor(s) David It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column L line 30, (pye--" should read (phe- Signed and sealed this 114.1311 day of May 197L|..

(SEAL) Attest:

EDWARD l I.FLETCIER,JR. Y c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PC4050 (10-69) 

2. The fluid of claim 1, wherein the anthraquinone is 2-benzylanthraquinone.
 3. The fluid of claim 1, wherein the anthraquinone is 2-(p-benzyl)benzylanthraquinone.
 4. The fluid of claim 1, wherein the anthraquinone is 1,4-diphenylanthraquinone.
 5. The fluid of claim 1, in which the polybenzyltoluene is a polystyrene-modified polybenzyltoluene.
 6. In a fluid, light-modulating medium for use in apparatus in which a thin layer of said fluid, light-modulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said plane to build up charge in said fluid, light-modulating medium, which charge produces self-erasing deformation in the surface of said layer, each said deformation acting to diffract light directed at said layer from a light source in a light optical systeM, the diffracted light being projected by the optical system as a function of each deformation to form self-erasing, rapid, decay images, the improvement wherein said fluid is the fluid of claim
 1. 7. In a fluid, polystyrene-modified polybenzyltoluene light-modulating medium for use in apparatus in which a thin layer of said fluid, light-modulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said plane to build up charge in said fluid, light-modulating medium, which charge produces self-erasing deformation in the surface of said layer, each such deformation acting to diffract light directed at said layer from a light source in a light optical system, the diffracted light being projected by the optical system as a function of each deformation to form self-erasing, rapid, decay images, the improvement wherein the fluid, polystyrene-modified polybenzyltoluene light-modulating medium is the fluid of claim 